Liquid crystal display having wide viewing angle range

Abstract

In a liquid crystal display having a wide viewing angle range, first and second retardation films each having negative optical anisotropy in which the refractive index in the axial direction tilted in a predetermined direction with respect to the normal line to the film plane are inserted between a liquid crystal cell including a pair of substrates and a front polarizer and between the liquid crystal cell and a rear polarizer, respectively, such that the direction components along the film planes of the axial directions are set along the aligning treatment directions of the liquid crystal cell. A third retardation film having refractive index anisotropy in the film plane is arranged between the first retardation film and the adjacent front polarizer such that the maximum refractive index direction in the film plane becomes parallel or perpendicular to the absorption axis of the front polarizer. The retardation difference generated by light that becomes incident on the liquid crystal display obliquely with respect to the normal direction is compensated by the third retardation film. Residual retardation in liquid crystal molecules near the substrate of the liquid crystal layer is compensated by the first and second retardation films. Hence, high-contrast display at a wide viewing angle can be obtained independently of the incident angle.

What is claimed is:  
     
       1. A liquid crystal display comprising: 
       a liquid crystal cell having alignment films formed on inner surfaces of a pair of front and rear substrates opposing each other, respectively, so as to be aligned in predetermined directions, and a liquid crystal layer formed between the pair of substrates;  
       a pair of polarizers arranged on the front side as an observation side of said liquid crystal cell and the rear side as an opposing side, respectively, so as to have absorption axes making predetermined angles with aligning treatment directions of the alignment films formed on the adjacent substrates;  
       a first retardation film inserted between said liquid crystal cell and said polarizer on the front side of said liquid crystal cell so as to have an axial direction, in which a refractive index is minimized, in a direction tilted in a predetermined direction with respect to a normal line to a film plane, and a direction component along the film plane in the axial direction, which is set along the direction of the aligning treatment of the alignment film formed on the substrate on the front side of said liquid crystal cell;  
       a second retardation film inserted between said liquid crystal cell and said polarizer on the rear side of said liquid crystal cell so as to have an axial direction, in which a refractive index is minimized, in a direction tilted in a predetermined direction with respect to a normal line to a film plane of the second retardation film, and a direction component along the film plane in the axial direction, which is set along the direction of the aligning treatment of the alignment film formed on the substrate on the rear side of said liquid crystal cell; and  
       a third retardation film having refractive index anisotropy in a film plane and arranged in at least one of a position between said first retardation film and the front polarizer and a position between said second retardation film and said rear polarizer so as to have a direction in which a refractive index in the film plane is maximized is substantially parallel or perpendicular to the absorption axis of said adjacent polarizer.  
     
     
       2. A display according to claim  1 , wherein each of said first and second retardation films comprises a discotic liquid crystal layer in which liquid crystal molecules are aligned to gradually rise from an aligned state along one surface, from the one surface toward the other surface of said film, and has the axial direction, in which the refractive index is minimized, in a direction along an average molecular axis direction of discotic liquid crystal molecules in an intermediate direction in a direction of thickness of said discotic liquid crystal layer, and the direction component along the film plane in the axial direction of each of said first and second retardation films is set parallel to the direction of the aligning treatment of the corresponding adjacent substrate. 
     
     
       3. A display according to claim  2 , wherein 
       said liquid crystal cell in which the direction of aligning treatment of the alignment film formed on the front substrate is perpendicular to that of the alignment film formed on the rear substrate has the twisted liquid crystal layer sandwiched by the substrate,  
       said pair of polarizers have the absorption axes parallel to the direction of aligning treatment of the alignment films formed on the adjacent substrates of said liquid crystal cell; and  
       said third retardation film has the direction in which the refractive index is maximized in substantially parallel to the absorption axis of said adjacent polarizer.  
     
     
       4. A display according to claim  3 , wherein said third retardation film is arranged between said front polarizer and said first retardation film. 
     
     
       5. A display according to claim  3 , wherein said third retardation film is arranged between said rear polarizer and said second retardation film. 
     
     
       6. A display according to claim  3 , wherein said third retardation films are respectively arranged both between said front polarizer and said first retardation film and between said rear polarizer and said second retardation film. 
     
     
       7. A display according to claim  3 , wherein for said third retardation film, if an s-axis represents the direction in which the refractive index is maximized in the film plane of the third retardation film, an f-axis represents a direction perpendicular to the s-axis in the film plane, and a z-axis represents a normal direction to the film plane, letting ns be a refractive index along the s-axis, nf be a refractive index along the f-axis, and nz be a refractive index along the z-axis, the refractive indices satisfy 
       a relationship 0<(ns−nz)/(ns−nf)≦1.  
     
     
       8. A display according to claim  7 , wherein a value of a product Δnd of a birefringence index and liquid crystal layer thickness  d  of said liquid crystal cell falls within a range of 300 to 500 nm, and letting d′ be a thickness of said third retardation film, said third retardation film has retardation with which 
       a value (ns−nf)·d′ falls within a range of 250 to 450 nm, and  
       a value (ns−nz)·d′ falls within a range of 70 to 450 nm.  
     
     
       9. A display according to claim  2 , wherein 
       said liquid crystal cell in which the direction of aligning treatment of the alignment film formed on the front substrate is perpendicular to that of the alignment film formed on the rear substrate has the twisted liquid crystal layer sandwiched by the substrate,  
       said pair of polarizers have the absorption axes parallel to the direction of aligning treatment of the alignment films formed on the adjacent substrates of said liquid crystal cell; and  
       said third retardation film has the direction in which the refractive index in the film plane is maximized in substantially perpendicular to the absorption axis of said adjacent polarizer.  
     
     
       10. A display according to claim  9 , wherein said third retardation film is arranged between said front polarizer and said first retardation film. 
     
     
       11. A display according to claim  9 , wherein said third retardation film is arranged between said rear polarizer and said second retardation film. 
     
     
       12. A display according to claim  9 , wherein said third retardation films are respectively arranged between said front polarizer and said first retardation film and between said rear polarizer and said second retardation film. 
     
     
       13. A display according to claim  9 , wherein for said third retardation film, if an s-axis represents the direction in which the refractive index is maximized in the film plane, an f-axis represents a direction perpendicular to the s-axis in the film plane, and a z-axis represents a normal direction to the film plane, letting ns be a refractive index along the s-axis, nf be a refractive index along the f-axis, and nz be a refractive index along the z-axis, the refractive indices satisfy 
       a relationship 0<(ns−nz)/(ns−nf)≦1.  
     
     
       14. A display according to claim  13 , wherein a value of a product Δnd of a birefringence index and liquid crystal layer thickness  d  of said liquid crystal cell falls within a range of 300 to 500 nm, and letting d′ be a thickness of said third retardation film, said third retardation film has retardation with which 
       a value (ns−nf)·d′ falls within a range of 250 to 450 nm, and  
       a value (ns−nz)·d′ falls within a range of 70 to 450 nm.